Look at the panel to the right under popular disucssions. One topic is how to change language.
this is for Swedish, but you can get the idea:
Simply navigate to this directory: C:\Program Files\Adobe\Adobe Photoshop CS5.1\Locales\sv_SE\Support Files
the sv_SE folder is named such because my version is Swedish. Just find the similar named folder that looks like it should correspond with your language.
In the support files directory there will be a file named tw10428.dat
Make sure that photoshop is closed, change the file extension to .bak instead of .dat, and then restart photoshop.
Your software will now be in English.